Semiconductor nanocrystals have been a subject of great interest, promising extensive applications including display devices, information storage, biological tagging materials, photovoltaics, sensors and catalysts. Nanocrystals having small diameters can have properties intermediate between molecular and bulk forms of matter. For example, nanocrystals based on semiconductor materials having small diameters can exhibit quantum confinement of both the electron and hole in all three dimensions, which leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of nanocrystals shift to the blue (i.e., to higher energies) as the size of the crystallites decreases.
The quantum efficiency of emission from nanocrystals having a core of a first semiconductor material can be enhanced by applying an overcoating of a second semiconductor material such that the conduction band of the second semiconductor material is of higher energy than that of the first semiconductor material, and the valence band of the second semiconductor material is of lower energy than that of the first semiconductor material. As a result, both carriers, i.e., electrons and holes, are confined in the core of the nanocrystal.